what's advantages of hi-index and polycarbonate lenses?

[Yogi Bear]

i'm curious as to why polycarbonate lenses are stronger and roughly the same weight as hi-index, but hi-index and mid-index lenses costs more? my understanding are that hi-index lenses are 1.6-1.7 index, poly are 1.6, the usual lenses are 1.5, and mid index lenses are about 1.5+. if so, why are the mid index lenses more expensive than the polycarbonate? i've tried out polycarb lenses a couple of years ago and i noticed a yellowish effect at the edges of white objects. is this distortion why poly lenses are less expensive? i'm gonna try some mid-index lenses for my next pair of glasses. just wanted some input.

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[rpames]

Basically hi index are quite a bit thinner. They make the curvature of the lens flatter, thus making the lens thinner. The reason it is so much more expressive is that this process is a pain in the butt to do. The lab has to use special calculations and equipment...it is a pain.

The yellow you are referring to is the "Abby effect" (I think that is what it is called). The poly lenses can cause a chromatic aberration that gets worse towards the edge of the lens. This is because the front curve is different then the back and this increases toward the edge. In the center of the lens, both sides are the same, so there is no aberration.

 

[OD2BMike]

It's a matter of terminology; poly is sometimes considered high index, but is usually less expensive than other high index materials. I've never seen poly priced below standard index glass, or CR39 plastic, but I guess it's possible. One reason that poly may be cheaper is that it is highly recommended or required in certain cases for it's shatter resistant properties. Maybe the retailer that you shopped discounts it for that reason. Also be aware that most high index lenses, including poly, have more distortion, or lower abbe value, than standard index lenses.

The refractive index is essentially a measurement of the materials' ability to bend light. The higher the index of a material, the more influence it has on light, so the thinner a lens material can be. Unfortunately, higher index materials tend to have lower "abbe values", i.e. more chromatic aberration. At the same time, polycarbonate manufacturing has improved in the last several years. You might want to give poly another try. You can find abbe value tables posted on the net, if this interests you. To me it has always seemed that 1.6 plastic is the best when you weight price, abbe value, and index. Hope this helps

 

[TomOD]

Mike,

Are you enjoying the VA? I imagine your seeing some interesting pathology. I may have told you this before: I have a contract with the local VA (they can't seem to keep a doc in house??). They send me around 50-80 pts. per month. Pretty crusty fellas in most cases but lots of interesting eyes and too many stories.

My only problem is that almost every one of them thinks they deserve 2 pairs of glasses and they both have to be progressive, transitions

Hope you learn alot.

Tom

 

[OD2BMike]

Nice to hear from you Tom.

The residency is going great. I must admit that so far this program has met and exceeded my expectations as a residency program. As you know Tom, VA patients are a unique breed in that most don't have a clue about how to live healthy. This has provided a great opportunity to learn the treatment and management of ocular and systemic pathology. I can't begin to talk about the variety of things we see and do here in such a short response.

We also find that most veterans feel they are entitled to 2 pairs of transition, progressive lenses. We do what we can I find that most patients are very friendly, and most are appreciative of the care we give. Most have stories. If anyone is interested in more info on this program, visit
www.oculardisease.com or email me.